Does that mean that most fonts already include normal East Asian characters, such as Japanese and Chinese, because they display fine along with English characters in the same file?

For example, the font Cambria.

I believe what I wrote above is true since Korean seems to only display in font series that end with Che (such as @Batang, Batang, and BatangChe), Tahoma, Arial Unicode MS Series, and Microsoft San Serif.

Kurouto
This is system depended, say "Courier New" on default Russian Windows XP install doesn't contain Asian characters. There need to install Asian language support to make Japanese and Chinese characters visible with "Courier New".